The present invention concerns a structural component made of hard material for a wristwatch and in particular a middle part for a watch case, a winding crown or bracelet links made of a material having a Vickers hardness greater than or equal to 1100 HV.
Metals are very widely used to make external parts for wristwatches because of their very advantageous properties: high mechanical resistance, high level of tenacity, ease of use and machinability, etc.. Their hardness is however less than that of ceramics, and experience has shown that external parts for wristwatches have poor resistance to scratches caused by certain very hard agents such as silicon dioxide found in dust, marble or even such. There thus results a relatively rapid deterioration in the aesthetic appearance of these types of parts.
In order to avoid these drawbacks, these external parts are currently made of hard metal, for example tungsten carbide, titanium carbonitride or even ceramics.
These materials are however difficult to shape and in particular difficult to machine by means of conventional machining techniques, which constitutes a significant limitation during manufacturing of external parts for the watchmaking industry or jewellery parts whose often complex shapes make the manufacturing cost thereof high.
In particular, the assembly of various external parts made of materials of this type, such as for example the assembly of the middle part and the back cover or of the winding crown on its stem, presents significant difficulties.
Swiss Patent No. 622 151 discloses an example of an assembly structure of a back cover to a middle part of a watch case made of corundum. The middle part includes a plurality of recesses obtained for example by ultrasonic machining, grinding or electroerosion. These recesses have a flat base and a slightly truncated lateral face and open into the lower face of the middle part. Threaded plugs are glued in the recesses of the middle part. Screws pass through the openings in the back cover and engage in the plugs into which they are screwed. This assembly technique is also used for making ceramic winding crowns.
This assembly technique has however numerous drawbacks.
It is very difficult to control the quantity of glue when it is applied into the recesses, given the relatively small gluing surface onto which the plugs are glued.
Moreover, it is well known that the film of glue deteriorate quite rapidly over time. This proves particularly inconvenient in the case of a plug glued into the recess of a ceramic winding crown onto which traction efforts are regularly exerted.
Further, the implementation of this assembly technique is all the more delicate the smaller the dimensions. In order to provide an indication, the diameters of the plugs used are of the order of a few millimeters.
The main object of the present invention is thus to overcome the drawbacks of the aforementioned prior art, by providing a structural component made of hard material for a wristwatch allowing assembly with other structural components of the same type, which is reliable, simple and economical to implement.
The invention therefore concerns a structural component made of hard material for a wristwatch, characterised in that it includes a hole whose cylindrical wall includes a threading which is integral with said component.
As a result of this new structure, the assembly can easily be achieved by screwing directly into the body of said structural component, which gives it improved reliability. Indeed, thanks to this structural component, it is possible to form an assembly with another component including a positive strong mechanical connection which, in particular, does not involve the phenomenon of adhesion.